This invention relates to a vacuum type breaker contact material.
Generally, necessary properties required by a vacuum type circuit breaker contact are as follows:
(1) high dielectric strength; (2) high-electric current can be interrupted; (3) small chopping current; (4) small welding force (welding tendency); (5) little waste; (6) low contact resistance; and the like.
However, it is very difficult for the contacts used in practice to have all of the above mentioned properties. Thus, the conventionally used contacts have the most essential properties, but some other properties are sacrificed.
For example, the conventionally used vacuum type breaker contact (hereinafter referred to as Cu-W contact) made of a sintered tungsten matrix (hereinafter referred to as W) impregnated with copper (hereinafter referred to as Cu) has a satisfactory dielectric strength, but chopping current is large and welding tendency is high.
The term, "welding tendency (or welding force)" used herein means the phenomenon of melting and welding between two contacts caused by Joule's heat determined by the value of electric current applied between the two contacts and the value of contact resistance therebetween when the two contacts are brought into contact with each other. This welding force is expressed by a force (kg) necessary to detach the two contacts.
The conventional Cu-W contact was prepared by infiltrating Cu into a W matrix sintered to a predetermined density by the powder metallurgy technique. The amount of Cu infiltrated is determined by the density of the sintered W matrix.
Generally, a notable metallurgical reaction is not recognized between W and Cu in a sintered W matrix infiltrated with Cu. That is, the performance of the Cu-W contact depends on the individual physical properties of W and Cu in the texture of the W skeleton in which Cu is dispersed. Accordingly, the performance of the Cu-W contact is greatly influenced by the particle size of W. For example, the smaller the particle size of W, the more uniform the copper dispersion, and therefore chopping current and welding force become lower.
However, the ratio of the largest value/the smallest value of tungsten particle size in the conventional contact material is more than 10, thus tungsten in the conventional contact material has various particle sizes. Also, the limitation of the size of W particles to 2 .mu.m or less is not executed since the infiltration of Cu is harder.